If you want to read a full length Asimov book, my personal recommendation is the End of Eternity. It has a rather unique take on time travel and functions well as a stand alone book. It has just been reprinted after being out of print for too long.

Foundation is his most well known novel and it also very much worth reading.

I can't find someone violating the copyright online with a quick Google, but Asimov's short story "The Last Answer" is also a good one with a different take on religion than "The Last Question".